The present invention relates to an electric pump for hydrogen circulation having a rotary shaft which is rotatably supported by a housing and driven to rotate by an electric motor and a rotor which is disposed in a pump chamber and rotated by the rotary shaft.
A fuel cell system in which hydrogen and oxygen are reacted to generate electric power has therein a hydrogen circulation path through which unreacted hydrogen gas (or hydrogen off-gas) which is not used in a fuel cell is supplied to the fuel cell. A pump is provided in the hydrogen circulation path for pumping the hydrogen off-gas.
For example, an electric roots pump is used as the hydrogen-circulation pump. The electric roots pump has a pair of pump rotors which is disposed in a rotor chamber which is formed in a housing of the roots pump, and an electric motor which is disposed in the housing for rotating the pump rotors. As the electric motor rotates, the pump rotors are rotated and hydrogen off-gas is drawn into the rotor chamber and then discharged therefrom. The hydrogen off-gas which is pumped by the electric roots pump is mixed with newly supplied hydrogen gas and resupplied to the fuel cell.
In the above fuel cell system, water which is produced in the process of electric power generation by the fuel cell is discharged from the fuel cell and introduced into the rotor chamber together with the hydrogen off-gas. For example, when operation of the fuel cell system is stopped under a low-temperature environment such as subfreezing temperature, the water then remaining in the rotor chamber is condensed and frozen, with the result that the axial end surface of the pump rotor may be stuck to the inner wall surface of the rotor chamber. In this case, there is a fear that the electric roots pump fails to be started in restarting the fuel cell system.
There has been proposed an electric roots pump which is capable of starting even when the axial end surface of the pump rotor is stuck to the inner wall surface of the rotor chamber (e.g. Japanese Patent Application Publication No. 2005-155409). Specifically, Japanese Patent Application Publication No. 2005-155409 discloses a conventional electric roots pump (actually a roots compressor) which includes a selector switch which is connected between a drive motor and its power source, a starter sensor for detecting whether the drive motor is started or not, a temperature sensor for detecting that the compressor is placed under a low-temperature environment and a controller. In the state of the roots compressor where the temperature sensor detects that the compressor is placed under a low-temperature environment, the controller sends out a forward-rotation command to the selector switch thereby to cause the drive motor to rotate in the forward direction. Furthermore, unless the starter sensor detects that the drive motor is started when the controller has sent out the forward-rotation command, the controller repeatedly sends out a reverse-rotation command and the forward-rotation command alternately to the selector switch. Accordingly, the drive motor alternately generates rotary torques in the forward and reverse directions, thereby breaking frozen water. Thus, the pump rotor is separated from the inner wall surface of the rotor chamber and, therefore, the drive motor can be started up.
However, when the torque generated by the electric motor is small in the roots pump of the above-cited publication, it takes a long time to break the frozen water. Thus, the start-up of the pump is delayed. To increase the torque of the electric motor for achieving quick start-up of the pump, the electric motor needs to be made larger in size and the size of the electric roots pump is larger, accordingly.
The present invention which is made in view of the above problems is directed to an electric pump for hydrogen circulation in which an electric motor generates a greater torque without being larger in size for achieving quick start-up in a state where a rotor is stuck to the inner wall of a rotor chamber due to frozen water.